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gossan
Gossan (eiserner hut or eisenhut) is intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral vein. In the ''classic'' gossan or iron cap all that remains is iron oxides and quartz often in the form of ''boxworks'', quartz lined cavities retaining the shape of the dissolved ore minerals. In other cases quartz and iron oxides, limonite, goethite, and jarosite, exist as pseudomorphs replacing the pyrite and primary ore minerals. Frequently gossan appears as a red ''stain'' against the background rock and soil due to the abundance of oxidized iron and the gossan may be a topographic positive area due to the abundance of erosion resistant quartz and iron oxides. Although most gossans are red, orange, or yellow, black gossans from manganese oxides such as pyrolusite, manganite, and especially psilomelane form at the oxidized portion of Mn-rich mineral deposits.
In the 19th and 20th centuries gossans were important guides to buried ore deposits used by prospectors in their quest for metal ores.〔Johnson, Joseph Colin Francis (1897) ''Getting Gold: A Practical Treatise for Prospectors, Miners, and Students'' Lippincott, Philadelphia, Pennsylvania, (page 15 ), 〕 An experienced prospector could read the clues in the structure of the gossans to determine the type of mineralization likely to be found below the ''iron cap''.
==Name==
The name ''gossan'' appears to come from Cornish miner's slang.〔"gossan" ''Century Dictionary and Cyclopedia'' Vol. III, page 2581 〕 It comes from the Cornish ''gossen'', from ''gōs'' meaning "blood", from Old Cornish ''guit''.〔"gossan" ''Oxford English Dictionary''〕 The terms "iron cap" and "iron hat", common in America,〔 are direct translations from the German "''eisenhut''", which term was also used in America.〔
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